A joint within the human body is a juncture between two or more bones or other skeletal parts. The ankle, knee, hip, elbow, shoulder, and fingers are just a few examples of the multitude of joints found within the body. As should be apparent from the above-listed examples of joints, many of the joints permit relative motion between the bones, such as a sliding, gliding, hinge, or ball-and-socket movements. For instance, the ankle permits a hinge movement, the knee allows a combination of hinge and gliding movements, and the shoulder and hip permit movement through a ball-and-socket arrangement.
The joints in the body are stressed or can be damaged in a variety of ways. For example, a gradual wear and tear is imposed on the joints just through the continuous use of the joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces directed to the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in direct contact with each other. In contrast to normal use, a trauma to a joint, such as the delivery of a large force, may cause considerable damage to the bones, the cartilage, or to other connective tissues, such as tendons or ligaments.
Arthropathy, a term referring to a disease of the joint, is another way in which a joint may become damaged. Perhaps the best known joint disease is arthritis, which generally refers to a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity. There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within the joint. Another type of arthritis is osteonecrosis, which is caused by the death of part of the bone due to a loss of blood supply. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis, destroy cartilage and are associated with inflammation of the joint lining.
The hip joint is one of the joints that is commonly afflicted with arthropathy. The hip joint is a ball and socket joint that joins the femur or thigh bone with the pelvis. The pelvis has a semi-spherical socket called the acetabulum for receiving a ball-shaped head of the femur. Both the head on the femur and the acetabulum are coated with cartilage for allowing the femur to move easily within the pelvis. Other joints commonly afflicted with arthropathy include the spine, knee, shoulder, elbow, carpals, metacarpals, and phalanges of the hand.
Arthroplasty, as opposed to arthropathy, commonly refers to the making of an artificial joint. In severe cases of arthritis and other forms of arthropathy, such as when the pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint with an artificial joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question but in general involves replacing a terminal portion of the afflicted bone with a prosthetic implant and inserting a member to serve as a substitute for the cartilage. The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and the cartilage substitute member is chosen to provide lubrication to the joint and to absorb some compressive forces. Suitable materials for the implant include metals and composite materials, such as titanium, cobalt chromium, and zirconia ceramic, and suitable materials for the cartilage substitute member includes polyethylene. A cement may also be used to secure the prosthetic implant to the host bone.
A total hip replacement, for example, involves removing the ball-shaped head of the femur and inserting a stem implant into the center of the bone, which is referred to as the medullary canal or the marrow of the bone. The stem implant may be cemented into the medullary canal or may have a porous surface for allowing the bone to heal directly to the implant. The stem implant has a neck and a ball-shaped head which are intended to perform the same functions as a healthy femur's neck and ball-shaped head. A polyethylene cup is inserted into the acetabulum and has a socket for receiving the head on the stem implant.
Unfortunately, some patients who have undergone partial or total joint replacement require revision surgery. Revision surgery may be required soon after the primary surgery or may not be needed for years. Revision surgery may be required for any one of a number of reasons. For one, fixation of the joint may become compromised. A trauma to the joint may have resulted in the loosening or the loosening of the joint may have been caused by other factors, such as an insufficient bond between the implant and the host bone. The loosening of an implant can be quite painful and also can pose a danger to the patient. For instance, movement of the loose implant within the bone may fracture the bone itself.
Revision surgery may be necessary for other reasons. For example, particle debris from the cartilage substitute member or even from any of the other implants may cause osteolysis in the patient. In general, osteolysis is the body's natural immune response to foreign objects which results in inflammation and pain in the joint. Another reason why revision surgery may be necessary is due to death of part of the bone. A bone requires stress in order to remain strong and a bone that is not stressed will become weak and fragile. The insertion of an implant may stress shield portions of the bone whereby these portions no longer receive the stress that it requires in order to remain strong. Another reason to perform revision surgery is to take advantage of advancements in prosthesis design. Thus, for any number of reasons, revision surgery may be necessary or desirable for a patient.
Revision surgery is more complicated than the primary partial or total joint replacement since it requires the removal of the previously inserted stem implant and the reintroduction of another stem implant. When the stem implant is removed, a significant portion of the surrounding tissue is removed along with the implant. If cement was used to secure the original implant, then additional tissue is normally removed to ensure that all of the cement has been removed from within the medullary canal. During a revision surgery, this void left by the removal of the surrounding tissue must be filled in order to allow the stem implant to bond with the host bone.
Dr. Ling et al. (collectively "Ling") have developed a surgical technique for the revision of a hip system. An example of this system is the CPT.RTM. (collarless polished taper) Revision Hip System, marketed by Zimmer, Inc. of Warsaw, Ind. Reference may also be made to U.S. Pat. Nos. 5,047,035, 5,192,283, 5,314,493, 5,326,376, 5,470,336, 5,683,395, 5,718,707, and 5,755,720 which are fairly representative of conventional approaches to revision surgery and to arthroplasty in general and which are incorporated herein by reference.
The CPT.RTM. technique involves the removal of the stem implant, any fibrous membrane, and bone cement and then thoroughly lavaging the femoral canal. A guide wire is threaded into a stiff intramedullary plug and the plug and guide wire are placed within the medullary canal with the plug firmly seated within the femur. The guide wire is inserted so that it is aligned with a longitudinal axis of the femur. A small amount of morselized allograft is inserted into the femur and packed until the distal one third of the proximal femur has been filled.
Next, a tamping process is performed which packs additional allograft material into the medullary canal until the planned size for the stem implant is reached. This tamping process begins with the insertion of a relatively large cannulated tamp approximately two sizes larger than the final component. A force is applied to the tamp, such as from a hammer, until the tamp is fully seated on the graft material. A smaller tamp size is selected and additional graft material is introduced into the medullary canal. A force is applied to this smaller sized tamp until it is fully seated on the allograft material. This tamping process is repeated with progressively smaller tamp sizes until the appropriate size is firmly seated. Thus, with this process, the allograft material is distally packed into the medullary canal with tamps of progressively smaller sizes until the appropriate size is firmly seated.
After using the progressively smaller tamps, final filling of the medullary canal is completed with proximal packers with the final tamp remaining in place. After proximal packing of the medullary canal is completed, the guide wire is removed and the tamp is left in place. Blood pooled at the distal end of the stem may be extracted with suction applied to the guide wire hole in the tamp. The tamp is then removed just immediately prior to insertion of cement into the medullary canal. A cement gun with a small diameter nozzle cut off to the length of the stem is used to inject cement into the narrow distal stem area. A second larger nozzle is then used to complete filling of the proximal femur and to pressurize the cement. The stem is inserted and pressure is maintained until the cement has polymerized.
The CPT.RTM. technique suffers from a number of the same limitations as the primary cases. Stems inserted with the CPT.RTM. technique, for example, have a fairly high rate of subsidence. The stem implant does not remain in place but rather migrates within the cement, which is excessive, results in loosening of the stem. A loose stem implant, as discussed above, is causes pain and may result in fracture of the host bone. Subsidence of the stem implant may also introduce a discrepancy in length between the limbs, which is especially serious if the differences in lengths are in the femur or tibia. A loose stem implant is also problematic since it increases the chance that particle debris from the cement is generated, thereby increasing the rate of osteolysis.
Another limitation of the CPT.RTM. technique is that the results of partial or total joint replacements vary considerably between surgeons due to variability in the packing of the medullary canal. At present, a surgeon has no objective indication as to whether the medullary canal is sufficiently packed with graft material. Consequently, even though surgeons may follow the same procedure, the amount of graft material introduced into the medullary canal and the density of the graft material varies between surgeons, with some surgeons more tightly packing the medullary canal than other surgeons. Some surgeons may be reluctant to pack the medullary canal as tightly as possible since in doing so the surgeon may inadvertently fracture the host bone. Even with a single surgeon, the results of a surgery will likely vary since the surgeon cannot accurately gauge the density and distribution of the graft material within the medullary canal.